In a wood pulping process, wood chips are fed into a digester. Typically, the digester is pressurized and operates at about 160-180° C. An aqueous solution, white liquor (typically comprised of NaOH and Na2S), is mixed with the wood chips. The white liquor or chemical pulping material neutralizes the organic acids in the chemical matrix of the wood. Lignin and other organic material, which contribute to about one-half of the mass of the wood, dissolve into the white liquor and exit the digester as weak black liquor. The remaining material, pulp, constitutes the wood fiber that is used in the papermaking process.
The weak liquor typically has a solids content of approximately 15% by weight, which is too low for combustion. To raise the solids content of the weak black liquor, the weak black liquor is typically concentrated in multi-effect evaporators until its solids content is approximately 65-85%. Thereafter, the concentrated weak black liquor is referred to as concentrated black liquor.
Many pulp mills employ what is referred to as the Kraft chemical recovery process. This process has three main objectives: (1) minimizing the environmental impact of waste material (black liquor) from the pulping process; (2) recycling pulping chemicals that form NaOH and Na2S; and (3) generating steam and power.
The Kraft chemical recovery process begins by directing the black liquor to a recovery boiler. Concentrated black liquor is sprayed into a lower part of the recovery boiler, where it is burned in an oxygen deficient environment so that sodium sulfide (Na2S) is formed. The inorganic sodium and sulphur are removed as molten smelt, which consists mainly of Na2S and sodium carbonate (Na2CO3). The molten smelt is directed to a dissolving tank, where it is dissolved in water to form what is referred to as green liquor. The green liquor is directed to a causticizing plant where it is reacted with lime, CaO, to convert the Na2CO3 to NaOH. The causticized green liquor is known as “white liquor,” which contains mostly NaOH and NA2S. It is returned to the digester for reuse in pulping. Precipitated CaCO3 (sometimes referred to as lime mud) from the causticizing reaction is washed and sent to a lime kiln, where it is heated to high temperature to regenerate CaO for reuse.
Chlorine (Cl), present in mills in the form of chloride, and potassium (K) are known to have a negative impact on the operation of chemical recovery processes in pulp mills. These elements, despite their small quantities in black liquor, can drastically lower the melting temperature of fly ash deposits and contribute to severe fouling and corrosion of heat transfer tubes in recovery boilers.
Chloride and potassium are concentrated in the ash formed during the combustion of black liquor in the recovery boiler. The ash mainly consists of sodium and potassium salts, wherein sulfate, carbonate, and chloride make up the dominant anions.
Presently most, if not all, of the precipitator ash collected and withdrawn from the recovery boiler is recycled to the black liquor to be burned in the boiler. When the concentration of the chloride or potassium becomes elevated, a portion of the precipitator ash is purged from the system.
As pulp mills have tightened their liquor cycle in recent years to improve spill control and decrease chemical losses, chloride and potassium concentrations in the mill liquor have increased, causing problems in recovery boiler operations. This has led to renewed interest in chloride and potassium removal.